If weeds are allowed to grow in a place such as a railroad, superhighway, open space left as it is, factory site, bank, levee between rice fields, orchard, or unplowed paddy or field, they presumably exert various bad influences on the place, for example, disturb the visibility of transportation, become a source of generation of disease and insect damage or interfere with farm work. In consideration of recent shortage in labor and increase in the labor cost, it has been difficult to control such weed growth, depending only on the human power and machine and it has been important to use a nonselective herbicide such as glyphosate, glyphosine, bialaphos or glufosinate.
When a lethal amount of the above-described nonselective herbicide is employed on the face of a slope or a levee between rice fields, however, it destroys almost all the weeds and no plant is left on the surface of soil, which leads to a problem of an outflow of the soil. In such a place, there is accordingly a demand for an agent that does not kill weeds completely but leaves various weeds green and retards their growth over a long period of time, that is, a plant growth retardant. Upon actual weed growth control, on the other hand, an amount of the agent used differs with the place to be sprayed owing to the influence of wind or the like. A plant growth retardant is, consequently, required to exhibit plant growth retarding effects at a wide range of amount of it used. In other words, there is a requirement for the development of a plant growth retardant which does not destroy the root of weeds even if the amount of it used is a little larger than its standard amount and at the same time and which exhibits plant growth retarding effects even if the amount of it used is a little smaller than its standard amount.
There have been attempts to use the above-described nonselective herbicide at a reduced amount of it or to use the nonselective herbicide as a plant growth retardant after adding thereto an agent for suppressing its herbicidal activity (Japanese Examined Patent Publication No. Sho 56-6402, Japanese Unexamined Patent Publication No. Sho 59-101500, Weed Science, 39, 622-628(1991)). Such attempts are however accompanied with the drawbacks that the plant growth retarding effects are not exhibited at a wide range of amount of herbicide used, the plant growth retarding spectrum is narrow, the plant growth retarding effects do not continue long and the like and, therefore, have not yet reached a practical use. There is accordingly a demand for the development of a plant growth retardant which can overcome the above-described defects.
In addition, there have been various attempts to develop a herbicide which has a wide herbicidal spectrum, has high safety for circumstances and shows selectivity on crops (European Patent EP 431545 and the like), but it has not yet been practically utilized. There is accordingly a demand for the development of a herbicide having wide herbicidal spectrum and excellent selectivity on crops.